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Abstract

This study investigates how accurately the interannual variability over the Indian
Ocean basin and the relationship between the Indian summer monsoon and the
El Nino Southern Oscillation (ENSO) can be simulated by different modelling
strategies. With a hierarchy of models, from an atmospherical general circulation
model (AGCM) forced by observed SST, to a coupled model with the ocean
component limited to the tropical Pacific and Indian Oceans, the role of heat
fluxes and of interactive coupling is analyzed. Whenever sea surface temperature
anomalies in the Indian basin are created by the coupled model, the inverse relationship
between the ENSO index and the Indian summer monsoon rainfall is
recovered, and it is preserved if the atmospherical model is forced by the SSTs
created by the coupled model. If the ocean model domain is limited to the Indian
Ocean, changes in the Walker circulation over the Pacific during El Nino years
induce a decrease of rainfall over the Indian subcontinent. However the observed
correlation between the ENSO and the Indian Ocean Zonal Mode (IOZM) is
not properly modelled and the two indices are not significantly correlated, independently
on season. Whenever the ocean domain extends to the Pacific, and
ENSO can impact both the atmospheric circulation and the ocean subsurface in
the equatorial Eastern Indian Ocean, modelled precipitation patterns associated
both to ENSO and to the IOZM closely resemble the observations.

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